We got an alarm system because my wife once came home to an intruder in the process of burglarizing the house. I installed an Ademco Vista 20p panel to the existing wiring, and added motion sensors and keypads. It's monitored by Alarm Relay http://alarmrelay.com
for less than $10/month and they've been very responsive. You can buy internet or cellular modules and also expand to wireless sensors.
Be sure to ask your homeowner's insurance company if they offer a premium discount. Our discount for central-station monitoring of burglary and fire just about covers the monitoring fee, so it made the choice easy.
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